Modern gas turbine engines which are used on aircraft are technically very complex machines. The high technology is especially apparent in the portions of the engine in which elevated temperatures are used. Not only are these components subjected to elevated temperatures, corrosion, and thermal fatigue, but they must be made as light as possible in design according to the dictates of their use in aircraft. Many of the components in modern gas turbine engines do have long lives; however, because of the hostile environment to which they are subjected, portions of a gas turbine engine have presented major maintenance problems. Since the components are rather costly because of the high technology used, repair or re-work of the engine is desirable over that of parts replacement.
One of the components which falls within the above context is the combustor of the engine. The combustor or burner is a portion of the engine where fuel is mixed with compressed air and combustion takes place. Increased performance levels of gas turbine engines can be obtained by increasing the operating temperatures thereof. In so doing the combustors of these gas turbine engines are exposed to extremely high temperatures and large variations in temperatures during engine operations.
As a result of the strenuous operating conditions, damage or deterioration in the form of cracks or other discontinuities including other openings can occur as from thermal cycling, impact from airborne foreign objects or their combinations. It has been observed that combustors can develop a number of small cracks while in or resulting from this strenuous service. Currently such cracks are repaired by welding. Sometimes repetitive repair welds are necessary in the same area because cracking also can occur during the repair operation.
Brazing is a common form of metal joining and consists of joining base metal surfaces by fusing a filler metal with a melting point depressant between the surfaces without appreciable fusion of the base metal surfaces themselves. The process is often an irreversible process with the filler metal or alloy forming an intermetallic solution with the metals being joined. Apart from compatibility with the base metal, filler metals are invariably selected for corrosion resistance in specific media and suitability for service at known operating temperatures.
Brazing is used to join sheet metal assemblies including combustor liners, combustor housings, air inlet ducts, cooling strips and others. A problem of damaging the brazed joints arises when welding near or on brazed areas of a combustor as the brazed area is relatively brittle. In fact, brazed components are currently considered to be nonrepairable by welding due to the embrittling effect of the braze materials on fusion welds. The reason for this embrittling effect is due to the use of melting point depressants which because of their nature cause the metal in which they are located to have a melting point less than that of the area to be welded such that during a repair operation when the temperature required for welding occur near or on the brazed areas, the brazed joints are more easily damaged.
As conventional repairs are carried out by welding, it would be advantageous to provide a combustor which can withstand weld repair. One alternative is to form the brazed connections by welding. However, this is not desirable as it would tend to increase manufacturing costs.
Accordingly what it needed is an improved method of forming brazed joints on combustors which overcomes the problems of the prior art. The present invention is directed to overcoming one or more of the above problems.